The present invention relates to positive working, photosensitive elements such as lithographic printing plates. More particularly, the invention relates to deletion fluids for such plates which are capable of removing unwanted photosensitive coatings from the surface of such plates.
Positive working lithographic printing plates are commonly formed by coating a substrate, such as an aluminum sheet with an aluminum oxide or anodic surface, with a photosensitive composition. Such composition generally comprises a quinone diazide photosensitizer in admixture with an alkali soluble resin such as a novolak.
Exposure of the composition through a photographic mask or positive flat to actinic radiation results in a solubility differential between the exposed and unexposed areas such that treatment with an appropriate developer results in removal of the radiation struck areas and retention of the desired image areas on the support. It is believed that such exposure decomposes the diazo structure in the radiation struck areas to form the free carboxylic acid group, thereby rendering the exposed areas soluble in akaline developer solutions. The developer solutions which are employed for this purpose can be aqueous solutions. Typical examples of alkaline developer solutions utilized heretofore in development of quinone diazide compositions include solutions of sodium hydroxide, lithium hydroxide, ammonium hydroxide, potassium hydroxide, sodium carbonate, sodium silicate or sodium phosphate.
Occasionally, after development, it is noticed that the plate possesses some imperfections. That is, some areas which should have been exposed and removed with the developer were actually not removed, or it is desired to remove some unexposed image area in a composition change. Clearly, the plate cannot be partially re-exposed to render such areas removable by the developer since inevitably some desired image areas would also be removed. It then becomes necessary to use a deletion fluid which is capable of being locally applied to dissolve and remove such undesired, non-alkali soluble areas.
There are many compositions which have been tried as deletion fluids heretofore, however, each has significant drawbacks. Some attack the underlying substrate, others do not desensitive the uncovered plate areas and thus attract ink during printing.
Generally, there are six requirements for a good deletion fluid. They are as follows:
(a) All coating components must be removed both quickly and easily.
(b) There is substantially no attack on the underlying substrate (i.e., dissolution of the anodic film). A neutral pH is preferred.
(c) The substrate must be hydrophilic after the deletion so as to be capable of giving clean reproductions, and, in the event of an image set-in, must provide acceptable adhesion.
(d) The product must be sufficiently viscous so as not to run into adjacent image areas. Further, it must be thixotropic so as to flow well in application.
(e) The solvent system of the product has to be non-toxic, have a high-boiling point, be miscible with water and be a solvent for all ingredients in the coatings. It would also be desirable to have an odorless system.
(f) The area of treatment must be easily rinsed of all deletion fluid and loosened coating.